1. Field of the Invention
The present invention relates generally to detection of a target nucleic acid sequence and specifically to early detection of preneoplasia or cancer in a subject by analysis of target mutant nucleic acid sequences in a saliva specimen from the subject.
2. Description of Related Art
An increasing body of evidence implicates somatic mutations as causally important in the induction of human cancers. These somatic mutations may accumulate in the genomes of previously normal cells, some of which may then demonstrate the phenotypes associated with malignant growth. Such oncogenic mutations may include a number of different types of alterations in DNA structure, including deletions, translocations and single nucleotide alterations. The latter, also known as point mutations, may frequently be involved in carcinogenesis, in that a variety of mutagenic chemicals induce such mutations. In addition, such mutations may occur spontaneously as a result of mistakes in DNA replication.
Advances in recombinant DNA technology have led to the discovery of normal cellular genes (proto-oncogenes and tumor suppressor genes) which control growth, development, and differentiation. Under certain circumstances, regulation of these genes is altered and cause normal cells to assume neoplastic growth characteristics. There are over 40 known proto-oncogenes and suppressor genes to date, which fall into various categories depending on their functional characteristics. These include, (1) growth factors and growth factor receptors, (2) messengers of intracellular signal transduction pathways, for example, between the cytoplasm and the nucleus, and (3) regulatory proteins influencing gene expression and DNA replication.
Point mutations have been directly implicated in the causation of many human tumors. Some tumors carry oncogenes of the ras gene family, which differ from their normal cellular counterpart proto-oncogenes by the presence of a point mutation at one of a limited number of sites in these genes. Similarly, point mutations in critical regions of tumor suppressor genes, such as p53, are often detected in tumor cells. These mutations represent qualitative changes in the tumor cell genome which distinguish these cells from normal cells and provide a basis for diagnosis of the genetic origin of a tumor under study. Identification of the mutations that have created active oncogenes may provide important diagnostic and prognostic clues for tumor development. For example, a number of mutations have been found to alter the 12th codon of the ras oncogenes, causing replacement of a normally present glycine by any of a number of alternative amino acid residues. Such amino acid substitutions create a potent transforming allele. Thus, the presence of a particular nucleotide substitution may be a strong determinant of the behavior of the tumor cell (e.g., its rate of growth, invasiveness, etc.). As a result, DNA probes for oncogene mutations have promise as diagnostic reagents in clinical oncology.
Among the various types of neoplasms, a number of those which are found in the lungs are associated with oncogenic mutations. Lung cancer is the leading cause of cancer related deaths in Western countries. The prognosis for patients with lung cancer is primarily dependent on the stage of the tumor at the time of clinical diagnosis. Currently, only 25 to 40 percent of all lung tumors are considered resectable at the time of initial assessment. Patients diagnosed early with stage 1 tumors have a 40-70% survival following surgical resection. An attempt at lung cancer screening through the use of tri-annual saliva cytology and annual chest x-ray has proven inadequate for the early detection of lung cancer. Alternatively, the finding that tumors progress through a series of well-defined genetic changes, including point mutations in oncogenes, has stimulated efforts to develop additional, non-invasive tests that could more reliably detect neoplasms of the lung. For example, U.S. Pat. No. 5,561,041 and U.S. Pat. No. 5,726,223 describe detection of neoplasia of the lung by analyzing sputum samples.
Other serious cancers are the head and neck cancers. Head and neck cancer remains a morbid and often fatal disease. Large tumor bulk and tumor extension are predictors of a local regional recurrence and poor outcome. Detection of occult neoplastic cells in surrounding surgical margins is a strong predictor of local regional recurrence resulting in a significant decrease in overall survival.
DNA contains unique sequences interspersed with moderately and highly repetitive DNA sequences. Variations in the repetitive sequence elements such as minisatellite (or variable number tandem repeat) DNA sequences and microsatellite (or variable simple sequence repeat) DNA sequences have been useful for chromosomal identification, primary gene mapping, and linkage analysis. Microsatellite DNA sequences are an especially common and highly polymorphic class of genomic elements in the human genome. One advantage to the use of repetitive sequence variations is the greater number of alleles present in populations compared with unique genetic sequence variations. Another advantage is the ability to readily detect sequence length variations using the polymerase chain reaction for the rapid and inexpensive analysis of many DNA samples.
Tumors progress through a series of genetic mutations. These genetic mutations can be used as specific markers for the detection of cancer. One set of genetic mutations that can be used to detect the presence of cancer is the loss of chromosomes. Diploid organisms, including humans, have pair of chromosomes for each member of the chromosomal set. Tumor cells will characteristically lose chromosomes, resulting in a single chromosome, rather than a pair of chromosomes, for each member of the chromosomal set. Chromosomal deletions and additions are an integral part of neoplastic progression and have been described in most kinds of cancers. A pair of chromosomes has two alleles for a genetic locus is heterozygous for that locus; therefore, the heterozygosity correlates to the cell having a pair of chromosomes. For years, these chromosomal deletions or amplifications were detected through the loss of heterozygosity.
Another of the genetic mutations used to detect the presence of cancer is genetic instability. Genetic recombination tends to occur most frequently at regions of the chromosome where the DNA is homologous (where the DNA has a high degree of sequence similarity). Where a DNA sequence is repetitive, the DNA homology is greater. The DNA homology occurs not only at the same genetic locus on the other pair of chromosomes, but also on other genetic loci or within the same locus on the same chromosome. Normal (non-tumor) cells tend to suppress this genetic recombination. Tumor cells, however, characteristically undergo increased genetic recombination. Where a DNA sequence is repetitive, genetic recombination can result in the loss of repeat DNA sequences or the gain of repeat DNA sequences at a genetic locus.
Microsatellite DNA instability has been described in human cancers. Microsatellite DNA instability is an important feature of tumors from hereditary non-polyposis colorectal carcinoma patients (Peltomaki et al., Science, 260: 810 (1993); Aaltonen et al., Science, 260: 812 (1993); Thibodeau et al., Science, 260: 816 (1993)). Microsatellite DNA instability by expansion or deletion of repeat elements has also been reported in colorectal, endometrial, breast, gastric, pancreatic, and bladder neoplastic tissues (Risinger et al., Cancer Res., 53: 5100 (1993); Had et al., Cancer Res., 53: 5087 (1993); Peltomaki et al., Cancer Res. 53: 5853 (1993); Gonzalez-Zulueta et al., Cancer Res. 53: 5620 (1993)).
Early detection of preneoplasia or cancer is crucial to increase the probability for treating such disorders. A non-invasive, simple screening test would provide health care workers with a tool for performing early diagnostic or prognostic screens.